Introduction to Ocean Sciences

Chapter 3: Studying the Oceans

Guide to Reading and Learning

Chapter 3 introduces you to the various component parts of the ocean environment and to the methods that ocean scientists have developed to pursue studies of the oceans. It answers one of the questions I have been asked most often—“Why do we not know more?” Understanding why we do not know more and learning about the difficulties ocean scientists face in observing the oceans will give you a much better appreciation of how the information presented in the rest of this text was obtained. It will also let you understand why there are so many references in the text to processes in the oceans that we do not yet understand very well.

The chapter describes just a few of the methods and instruments used by ocean scientists to obtain data to describe the topography of the sea floor, to sample and study the sea floor sediments, to obtain water samples to study the chemistry of seawater, to measure the temporal and spatial variations in temperature and salinity (salt content) of ocean water, to measure the speed and direction of ocean currents, and to sample and study the living organisms in the ocean. Some of the methods described are no longer used but provide historical perspective and explain why newer technologies based on satellites, sophisticated computerized instrumentation, and computer analysis of data have been key in dramatically accelerating our ability to study the oceans over the past several decades.

After the discussion of the methods of sampling sediments, water, and living organisms, the chapter then reviews some of the more recent advances including scuba, manned and unmanned submersibles, satellites, and computers and modeling. Data obtained from these systems has completely revolutionized ocean sciences but you may be surprised to learn that manned submersibles have played only a very minor role and will likely be relied on even less in the future. The future of ocean science lies in robotic observations in very much the same way as space and planetary exploration relies primarily on unmanned satellites and landers.

When you reach the subject of satellite sensors, take advantage of the Internet to browse some of the data that they are generating each day. Examples of the kinds of timely information you may find are the latest month’s global sea surface temperature map, the latest El Niño predictions, and analyses of sea surface temperatures and atmospheric pressures in the tropical Pacific, analysis and prediction of the latest wave height conditions in different coastal areas, and analysis of the Gulf Stream location. None of these types of measurement and prediction were possible just a few short years ago.

Chapter 3 Essential to Know 

Critical Concepts used in this chapter

CC.1, CC.6, CC.10, CC.14

 
3.1 The Unseen Domain

  • The major topographic features of the ocean floor were unknown until as late as the 1950s, and the topography of the ocean floor is still so poorly mapped that most topographic features less than about 10 km in width may have been completely missed.

3.2 Bathymetry

  • Measurement of the ocean water depth is known as bathymetry.

      Soundings     

  • The topography of the sea floor was hardly known at all until a few decades ago because of the difficulties inherent in measuring ocean depth before sonar was developed in 1920. Prior to that time all ocean depth measurements, or soundings, were made by lowering a weight on a rope or cable until it hit the seafloor and measuring the length of rope or wire payed out.

      Sounding Errors and Problems

  • Wire and rope soundings were tedious and subject to errors caused by difficulties in determining when the weight reached the seafloor. Errors also occurred when winds moved the vessel across the surface or currents moved the weight laterally so that the lowered line did not descend vertically.

      Echo Sounders

  • Sonar, or echo sounding, uses the travel time of a sound pulse from a vessel to the seafloor and back and knowledge of the salinity and temperature (which control the sound velocity) to measure seafloor depth along the line of the vessel’s travel.
  • The sound beam angle, although small, spreads the beam so that return echoes come from a substantial area of seafloor, especially in deep water. This prevents the sonar record from revealing detailed features of the seafloor such as narrow valleys or small hills.
  • The first comprehensive map of the ocean floor was produced in 1959 by compiling millions of kilometers of sonar and navigation data recorded on paper charts. Computer technology that could collect sonar data and plot the data on maps was not developed until much later.

      Wide-Area Echo Sounders

  • In the 1960s two different sonar systems were developed to create three-dimensional maps of the ocean floor using computer analysis of the echoes received. These systems utilize either multiple sound beams or wide-angle sound beams to scan a broad area under and to the sides of the vessel’s track.
  • Wide-area echo sounders are now being used to map the seafloor at unprecedented levels of detail. However, they are expensive and it will take many years or decades before even a substantial percentage of the ocean floor has been mapped by these techniques.

      Ocean Topography from Satellites

  • Very precise measurements of the mean level of the ocean surface made from satellites can reveal seafloor topography because rocks and sediments have higher density than water. As a result, elevated areas of the seafloor have greater mass than the surrounding seawater at the same level (same distance from the Earth’s center) and thus a greater gravitational attraction on the overlaying water, which produces a small elevation in the mean sea level. The entire ocean floor has been mapped by this technology but satellite sensors cannot resolve detail of the sea surface level, and thus seafloor topography, that is less than about 10–15 km in width. Therefore only major features can be mapped with this technology.

3.3 Seafloor Sediments

  • The seafloor is covered by sediments that range in thickness from zero to several kilometers.
  • Two types of sediment samples are important: samples of long vertical sequences of layers that contain the history of environmental conditions at the time each successive layer was deposited, and large samples of the top few tens of centimeters of sediment within which most organisms that inhabit ocean sediments live.

Grab Samplers and Box Corers

  • Samples of the surface layers of the sediments are usually collected with grab samplers or box corers. There are many designs but grab samplers are usually a sealed metal container with two halves separated at the top hinge like a clam shell. The sampler is lowered with the clamshell jaws open until they sink into the bottom.  The jaws are closed mechanically as the grab is pulled back up out of the sediments. Box corers consist of a weighted box with an open lower side and can be much larger than grab samplers. The box sinks into the sediments and is then closed off by a blade that slices under the box.

Gravity and Piston Corers

  • Long sequences of sediments layers (up to about 50 m) are collected with corers that resemble an apple corer—a long hollow tube is pushed into the sediment by weights, closed at the bottom by one of various types of closure mechanisms, and pulled back out of the sediments on a cable.
  • Gravity corers generally take shorter cores,and piston corers take longer ones. Unlike gravity corers, piston corers have an internal piston that is mechanically retracted as the main core barrel sinks into the sediment. This creates a suction effect that helps the corer to penetrate further and reduces disturbance of the sediment layers.
  • Both grab samplers and corers often fail to collect sediment samples because they hit the seafloor at an angle, they hit rock and not sediment, or their closure mechanism fails. This can waste many hours, as lowering and retrieving the sampler in deep water is a lengthy process.

Drilling Ships

  • Since 1978 a series of ships have been dedicated to drilling long core samples in the seafloor. When the drill string is retrieved (e.g., to change cutting heads) and reinserted, the string must be lowered through the water column and relocated in the same hole. As a result these ships are technologically complex and very expensive. However, data from drill cores was instrumental in confirming the plate tectonic theory and provides a record of historical climate changes.

Seismic, Magnetic, and Gravity Studies

  • The seafloor can be studied remotely from a surface vessel with seismic (sound) waves reflected off buried interfaces between layers, and by their magnetic properties and gravity.
  • Seismic profiling can be done by measuring the sound reflected off interfaces between sediment layers or by sound refracted within the sediments and layers and then scattered back upward.
  • Three-dimensional seismic profiling can produce detailed three-dimensional images of ancient reefs and oil and gas deposits buried under layers of sediments. Three-dimensional seismic profilers use an array of multiple sound sources and multiple hydrophones (listening devices), and the data can only be analyzed by very powerful computers.

Dredges

  • Dredges are used to collect samples of the seafloor in areas where there are no sediments, or when samples of nodules lying on or just under the sediment surface are required. These dredges usually consist of a rectangular metal frame that is dragged across the seafloor with an attached chain bucket. The bucket collects larger material while sediments pass through the chain mesh. Dragging large dredges across rocky seafloor can require great force to break the rocks and prevent snagging, so very thick cables must be used. Only the largest research vessels are able to use most dredges.
  • Submersibles and ROVs are now often used to collect seafloor rocks but these are more expensive to operate than dredges.

3.4 Chemical and Physical Oceanography

Sampling Bottles

  • Collecting water samples from deep within the oceans is tedious. The process requires that sampling bottles be lowered into the depths, where they are closed before retrieval. There are many designs of bottles with different closure mechanisms

Avoiding Sample Contamination

  • Concentrations of many chemicals in seawater are so low that contamination from the sea surface and sampler material is often a problem. Contamination comes from many sources including the metals of the wire used to lower the samplers, metals and other materials that comprise the sampler, and the surface microlayer.
  • To avoid contamination, especially from the ship’s atmosphere, hull, and oily discharges, as well as the surface microlayer, samplers are now made primarily of plastics and are often cleaned and sealed carefully in a clean room on board before being lowered into the ocean.. Only when several meters below the surface and away from these sources of contamination are the bottles opened and then lowered to their sampling depth. (Opening the bottles before they reach the desired depth prevents them from being crushed by the pressure.)

Determining the Depth of Sampling

  • Because wires never descend exactly vertically beneath a ship, measuring the depth at which the sampling bottle is closed is difficult. Modern systems use electronic pressure sensors that send data through a conductor in the cable.
  • Electronic pressure and water temperature sensors must be calibrated by reversing thermometers. Two thermometers are used, one that is open to outside pressure and one that is sealed in a protective envelope, The thermometers are designed to be turned upside down at the sampling depth; this breaks the mercury column so the amount of mercury remaining in the new lower half of the thermometer is a precise measure of the temperature at the time of reversal. The unprotected thermometer shows a temperature that is too high by an amount that is equal to the extra mercury squeezed out of the thermometer bulb by the pressure at the sampling depth.

Instrument Probes and Rosette Samplers

  • Salinity and temperature are the two most important properties of seawater and are routinely measured because they can be used to calculate water density. Salinity can be determined by measuring the electrical conductivity of seawater, and temperature can be measured by electronic “thermometers.”
  • Sensors that measure these properties can be mounted on a CTD (conductivity, temperature, depth) and can make continuous measurements as they are lowered and retrieved through the water column.
  • Water sampling bottles are often mounted in a ring (or rosette) around the CTD and closed individually by electronic signals sent down the wire when the sample bottles are at depths selected by scientists viewing the continuous temperature, salinity, and depth readout from the CTD.
  • There are very few seawater properties other than these that can be continuously monitored in situ by sensors.

 Measuring Currents

  • Currents are normally measured by three approaches: passive devices, current meters, and remote sensing systems.

Drifters, Drogues, and Floats

  • Currents can be measured by devices that float along with the current and either report their position as they move or can be followed to determine their changing position.
  • The simplest systems are drifters, cards that float on the water surface, or weighted drifters that sink just to the seabed but extended upward enough that they are carried along the seabed by currents. The drifters are washed up on beaches and, if returned to the researcher with information on where they were found, can reveal the long-term mean currents.

Mechanical Current Meters           

  • Mechanical current meters remain fixed in place and use a propeller, rotor, or inclinometer to measure the velocity of water flowing past, usually recording the data internally for analysis when the meter is retrieved.
  • Mechanical current meters are often deployed for several months as a string of meters at depth intervals along a mooring wire.

Acoustic Current Meters

  • Currents can be sensed remotely by bouncing sound waves off particles in the water and observing the Doppler shift (the change in sound echo frequency with speed and direction of a moving particle) due to the particles’ motion.
  • Meters can be mounted on a ship, on the seafloor, or in mid water on a mooring. Typically they can sense currents within about 100 m.
  • Acoustic tomography is performed with an array of sound transmitters and receivers using the same Doppler principle and powerful computer analysis.

3.5 Living Organisms in the Sea

  • Nets, traps, water samplers, and sediment samplers are used to collect biological organisms.

Special Challenges of Biological Oceanography

  • Biological sampling is complicated by the ability of some species to evade capture, the widely dispersed nature of biological populations, the range of sizes of organisms from viruses to whales, and the inability of many species to survive capture or the warmer low-pressure conditions in a sample after it is brought to the surface.

Nets, Water Samples, and Traps  

  • A variety of nets with different designs and mesh sizes are used to collect marine species of different sizes from the largest fishes to small plankton. Large fishes and invertebrates that avoid nets are often caught with lines and lures or by using a variety of traps. The smallest organisms are too small to be caught by nets and are, instead, collected by collecting water samples.

Fragile Organisms

  • Fragile organisms that are damaged or destroyed by nets and other samplers are collected by scuba divers in shallow waters and by submersibles in deep waters. Organisms are guided or gently sucked into a sampling container.

Migrations and Behavior

  • Migrations and behavior of marine animals are very difficult to study, as scuba divers can only spend very short periods of time underwater and cannot swim fast or far enough to follow many behaviors.
  • Recently developed miniaturized sensor packages can be attached to marine species to radio their data back to shore. They can transmit their data when the animal is at the surface, detach after a period of time to be recovered, or float to the surface and send their data. Some new sensors also allow a continuous data stream to be sent acoustically to underwater listening stations.
  • Sensors attached to marine animals can record various parameters including temperature and depth and can include sound recorders or video cameras. However, they cannot record precise locations when beneath the surface because GPS does not function below water.

3.6 Scuba, Manned and Unmanned Submersibles

  • Many scientific sampling and observation tasks can best be done when the scientist is able to see the sampling site or study animal.

Scuba and Habitats

  • Scuba has proved to be invaluable for ocean research, especially in coastal regions. However, scuba is restricted to shallow depths (generally less than about 100 m) and divers can stay underwater for only short periods of time. Underwater habitats can extend the amount of time per day that researchers spend at a study site but are very expensive, limited only to shallow depths, and require inhabitants to spend periods of days in a decompression chamber after living in the habitat.

Manned Submersibles

  • Manned submersibles have their place in ocean research but the extremely high costs of building and operating submersibles, and their limited range and endurance (typically they can dive for no more than a few hours), severely limit their use.

Remotely Operated Vehicles

  • Unmanned ROVs (remotely operated vehicles) and AUVs (autonomous underwater vehicles) are becoming progressively less expensive and more capable. They now perform an increasing percentage of all sampling and observation tasks in the oceans. ROVs can allow several scientists to see exactly where and what they are sampling from the ship’s laboratory at a drastically lower cost than manned submersibles and with greater safety.

3.7 Satellites

  • Satellites are the only means available to obtain synoptic measurements that show the distribution of properties across large areas of oceans at one time, or the variations of these distributions over time periods of days and months.
  • Satellites are limited in their ability to sense ocean properties because most electromagnetic radiation is absorbed in the upper layers of the oceans. Thus, most sensors that can be carried by satellite can only see surface or near-surface properties and cannot see into the depths.
  • Satellites that sense gravity variations or very small variations in the sea surface height can be used to make indirect observations of the sea floor topography.
  • Navigation satellites have proved especially useful for ocean sciences, as they now allow a ship’s position in the open oceans far from land to be determined to within a few meters.

3.8 Computers and Modeling

    • Most ocean science is now heavily dependent on high-powered computing capability to manage and analyze the massive amounts of data that are generated by satellites and other sensors.
    • Computer modeling is also necessary to study ocean processes, as these processes are complex with many interacting factors.
    • GIS (Geographic Information Systems) are computerized software systems that store, analyze, and display data for many different parameters, each referenced by a three-dimensional space coordinate and a time. They have become increasingly useful in analyzing and displaying large, multiple-parameter ocean data sets.

     

    Critical Concept Reminders:

    CC.1 Density and Layering in Fluids (p. 54)

    • Water in the oceans is arranged in layers according to water density. To read CC1 go to page 2CC.

    CC.6 Salinity, Temperature, Pressure, and Water Density (p. 52)

    • Sea water density is controlled by temperature, salinity, and to a lesser extent, pressure. Density is higher at lower temperatures, higher salinities, and higher pressures. Movements of water below the ocean surface layer are driven primarily by density differences. To read CC6 go to page 16CC.

    CC.10 Modeling (pp. 53, 65)

    • Complex environmental systems, including the oceans and atmosphere, can best be studied by using conceptual and mathematical models. To read CC10 go to page 26CC.

    CC.14 Photosynthesis, Light, and Nutrients (p. 58)

    • Chemosynthesis and photosynthesis are the processes by which simple chemical compounds are made into the organic compounds of living organisms. The oxygen in the Earth’s atmosphere is present entirely as a result of photosynthesis. To read CC14 go to page 46CC.

     

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